const babel = require('@babel/core');
const sourceCode = `
<h1>
  hello<span style={{ color: 'red' }}>world</span>
</h1>
`;

const result = babel.transform(sourceCode, {
    plugins: [
        ["@babel/plugin-transform-react-jsx", { runtime: 'classic' }]
    ]
});
console.log(result);

React.createElement("h1", null, "hello", React.createElement("span", {
    style: {
        color: 'red',
    }
}, "world"));